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How do you realign a directional gyro in an area of magnetic unreliability?
The primary means of determining heading in most small aircraft is the directional gyro, as this instrument is unaffected by the magnetic dip errors that cause a magnetic compass to give the wrong heading when at a nonzero angle of bank.  However, due to the rotation of the earth under the aircraft, the aircraft's movement over the curved surface of the earth, and friction within the gyro assembly, the directional gyro has to be periodically realigned, using the magnetic compass as a reference.
All well and good, but there are large parts of the earth where a magnetic compass cannot be trusted, because the horizontal component of the earth's magnetic field is too weak or erratically variable.  Examples of such areas of magnetic unreliability include the areas around the earth's magnetic poles (for instance, northern Canada), where the magnetic-field lines go almost straight up-and-down rather than side-to-side, and areas with large localized changes in magnetic deviation (for instance, western Lake Superior, where all the iron in the ground causes magnetic compasses to read almost 90° off in places).  Additionally, these areas go by true heading rather than magnetic (and the two differ considerably at high latitudes), so you can't even extrapolate a magnetic heading that you obtained before the compass became unreliable - you need to realign the gyro to true north as soon as you enter the area of magnetic unreliability.
Big airliners have GPS and IRS to give them an accurate measure of true heading and keep the directional gyro honest, but, if you're flying a small general-aviation plane not equipped with either, how do you correct the directional gyro's drift if you're somewhere your magnetic compass isn't reliable (especially if you're IFR and can't use ground landmarks to find where north is)?  If you're departing from within an area of magnetic unreliability, how do you align the directional gyro in the first place; if you're just entering a magnetically-unreliable area, how do you realign the gyro from magnetic to true?

You initially align it while on the ground, using a known reference like the runway.  While in flight, you'll need to use an astrocompass or sextant, if you have one and know how to use it and can see the sun or stars.
If you're a bush pilot chugging along VFR in your Beaver, you can make an eyeball alignment by judging heading against the apparent alignment of the airplane itself with landmarks. For example, there may be a feature like a river or shoreline along your route that has a straight section with a true track you can measure on your chart, so when you come to it you can align yourself with the feature below you and set your DG.  If you're flying along the Alaska Highway in your 172, it's easy to keep the DG aligned using the track of the highway below you (If you're flying your 172 to Alaska, you'll be sticking close to the highway anyway if you have half a brain).
Aside from tricks like that, there's not much you can do.  If you've measured the rate of precession of your DG, you could just apply an equivalent correction at a suitable interval. For example, if you know that your DG precesses 5 degrees in 20 minutes, you just apply a 5 degree counter adjustment every 20 minutes. It'll keep the DG roughly in the ballpark, good enough for VFR flying, but in any case a lot of the time you are going to be navigating by pilotage (landmark to landmark).  My bush flying wasn't in the far north, but still I rarely flew precise headings, just found my way from lake to lake, pointing the nose in the general direction I wanted to go and tracing my position/progress on a 50,000:1 scale topographical map, sometimes at 300 ft agl under a 500 ft cloud deck.
I wouldn't want to be doing an IFR flight on DG heading information alone unless I could keep the DG in alignment with an astrocompass/sextant (where you're going to need to get above cloud layers).
Up until about 15 years ago, knowledge of how to take a star or sun shot with a sextant and work out position and heading was a requirement for the Canadian ATPL license, but it was dropped.

What do you do when aligning the directional gyro before departure from a bush site without an obvious heading reference (given that it's probably easier to identify things on your map from the air than from the ground)?  Would that be a situation requiring the sextant in order to figure out which way you're going?

Not that I'm complaining; great answer!

Thing about bush operations, not many of them are operated in conditions requiring accurate compass headings, as they are mostly VFR/map navigation (or IFR as in I Follow Roads). The ones requiring proper navigation are flown with equipment not relying on gyros (well other than solid state gyros).

If you're on a lake you will just identify shore line land marks, work out the true direction between them on your chart, taxi adjacent to one, then point the nose at the other, and set the DG.  The times I needed a heading reference was when scud running in low vis down on the deck running from lake to lake, and I would leave one lake on a best guess particular heading and hold it until I hit the next lake, maybe 3-5 minutes later.  I was flying where the compass worked fine.

@JohnK: Was your best-guess heading in those situations ever off by enough that you missed the next lake entirely?

At the close distance between them with the size of the lakes you would hit it if you were +/- 15-20 deg as long as the lakes were too far apart.  Sometimes if it meant crossing an area with no features that went for 20 miles, or detouring to follow a chain of lakes or a river, I would detour to follow the chain of lakes.  Flying at 3-500 ft agl under a cloud deck with nothing but forest and rocks under you, it is very easy to get lost if the lake you are expecting to loom out of the murk doesn't appear when you expect it, so you tend to lean on following features rather than dead reckoning.


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What is the effect of aileron deflection on the centre of pressure?
Deflecting an aileron will increase or decrease the camber of the outer wing where the aileron is attached. I learnt on this platform that centre of pressure (c.p.) moves fore and aft with aileron deflection.
Is the c.p. movement as a result of aileron deflection significant?
If it is significant, how does one calculate the difference in c.p. with respect to aileron upward or downward deflection?

Probably you refer to ailerons used as flaperon when flaps are extended. Yes in this case the c.p. displacement is comparable to what happens with flaps. Calculations are beyond my scope but extremely complicated without much much more information and data

I think you forgot a link in along with the sentence "I learnt on this platform". Which platform are you talking about?

The CP of local sections where the ailerons are deflected should shift aft. In an asymmetric deflection case, very little pitch moment would be generated. For symmetric deflection (e.g. maneuver load alleviation, flaperon), there will be a net pitch moment, but should be small due to the outboard location of the panels, unless the wing is highly swept.
To estimate the effect, you can view ailerons as part-span plain flaps. You can use any of the semi-analytical methods from ESDU, DATCOM or Roskam. You can also use any vortex lattice method to get the answer. The estimates should agree fairly well for small deflections and low Mach.

Thank you for your answer,I'm trying to design an ultralight and can I safely conclude from your answer that the  movement of c.p  caused by  deflected ailerons are negligible in a slow,light weight,low performance ultralight airplane?@Jimmy

@DavidTeahay If you're talking about asymmetric aileron deflection, there should be very little pitching moment. Is that what you're ultimately interested in?

Yes sir, that's what Im ultimately interested in.

source: Stack Exchange

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When in 1982 did ICAO start using three-letter airline codes?
All airlines have an alphabetic identification code issued by ICAO.  From 1947, when ICAO was formed, until 1982, ICAO codes were two letters long;1 in the latter year, due to the increasing number of airlines in the world, ICAO lengthened its airline codes to three letters (although the preexisting two-letter codes remained valid until 1987).
Does anyone know when in 1982 ICAO introduced three-letter airline codes?  (I'm writing about an accident that occurred in 1982, and I can't tell if I should list the two-letter or three-letter ICAO designator for the flight.)

1: When IATA got into the business of issuing airline codes in the early 1950s, its members adopted their preexisting ICAO codes as their new IATA codes (this is why IATA codes are two characters long2).
2: Although the IATA system makes formal provision for three-character airline codes, it has never actually issued those, because airlines don't want to have to replace ancient equipment that can't handle airline codes longer than two characters.

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Are the CAA and FAA Class 1 medical certificates equivalent?
Is the CAA medical class 1 certificate equivalent to the FAA class 1 medical certificate? How so?
Or does one have to convert to the FAA license in order to achieve the medical certificate?

Welcome to aviation.se it a but unclear what you are asking. FAA Medicals are not licenses they are certificates issued by an AME that allow you to obtain a license such as a PPL (medical certs are no longer needed under the basic med rules for PPL but many have them), CPL or ATP. You will need an FAA medical certificate prior to getting a COP or ATP.

Sorry, I meant the certificates. Are the CAA medical certificates equivalent to FAA certificates?

I'm not sure what you're asking, but if it's whether you can use a CAA medical in place of an FAA one to get an FAA pilot's certificate, this question might be helpful, or maybe this one.  Otherwise, please tell us in more detail what you mean by "equivalent".

The CAA medical (UK or any other country with a CAA) is not equivalent to an FAA medical, you have to get an FAA medical to get a FAA license. The classes are similar but they cannot be used in place of the other. In the UK you can get an FAA medical from several AMAs.


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What is the function of interplane strut of a Biplane?
What kind of load does the interplane strut(s) carry,is it compression, tension or shear?
In order words,what's their function and how do you calculate the force that the interplane strut carries?

As Koyovis's answer shows, you simply have to calculate the vertical lift load acting at the interplane connection point to the lower wing panel to determine the compression load on the interplane strut, which will be somewhere around half or a bit more or less of the panel's total lifting force depending on the spanwise location.

Could you crop that image so it doesn't waste so much space above and below?

The wing strut prevents the lower wing from bending upwards due to the lift forces.

The biplane is a truss structure: rectangular shapes are "crossed out" by diagonal structural members, in order to create triangular shapes with only tension and compression in their structural members, no bending. If the diagonal is a rod, it can transfer both compression and tension forces and only one diagonal is required. If it is a cable, it can only transfer tension forces, and the other diagonal needs to be crossed out by a cable as well.
In flight, lift wants to bend the wings upward and lengthen the diagonal, which becomes tension loaded. The upper wing is then prevented from bending upwards and becomes compression loaded.
The interplane strut connects the lower wing with the upper wing, and prevents the lower wing from bending upwards. It is compression loaded: the diagonal exerts a downward force, lower wing lift exerts an upward force.
The method of how to compute the force is in this answer. In a truss structure the intersection points are modelled as hinges, and the structure members are dimensioned to carry the resulting tension/compression loads.

Minor quibble, but on biplanes with wire bracing it's a pin joint structure so the dotted lines should really be straight.  The curve implies root fittings able to take bending, like on a Fokker DR-1.

@JohnK the dotted lines are what the wings would do if there was no diagonal structural member.

the root fittings on biplanes are pin joints that can't take any bending.  If you took out the diagonal member, it would fold up like a parallelogram.

@JohnK I know. It's a truss structure.

Read my first comment carefully.

I think @JohnK is saying that if the diagonal member weren't there, the structure would bend at the joints, not by bending the wings. The shape would still be a parallelogram, just one with no horizontal edges. I don't think that's quite true though: the biplanes I know with wire bracing have each wing as a single member end-to-end, not separate left and right wings. Although the pin joints at the wires are (fairly) free to rotate, the wing roots are rigid, so the wings would still bend before the structure crumples.

@Dan Hulme Yes that is the case, for the top wing, if a biplane has a one piece wing so it's a beam from tip to tip. However that's not very common and most biplanes have a separate center section and single bolt joints from center section to upper wing panel, and fuselage to lower wing panel.  They are true pin jointed parallelograms. The Fokker DR-1 Triplane did have cantilever spars for all three wings however, and the spar roots were in pure bending, because they didn't have flying wires at all, and the interplane struts were only there because the German Air Ministry insisted on them.

This feels like a good reminder that wing structures are complex with many different fine details in various design options. - Vertical struts may at times carry a dynamic load as the end of a box-beam structure in some cases, transferring both compression and tension loads. [Wing loading can get complex, especially if a structure is approaching a 'rapid unplanned disassembly' state due to 'unfavourable conditions']

@DanHulme Indeed, that is how box beam structures are modelled,, with the intersection points absorbing zero torque and therefore acting as hinges.

It carries compression. The compression force is the share of the lower panel's lift being borne by the interplane strut's connection to the lower wing.  So to calculate the compression load for, say 1G flight at gross weight, you simply need to determine the lift force present at the interplane strut's attachment based on the wing panel's loading and the attachment's location in the lift distribution of the panel.  It's going to be somewhere in the range of half of the total lift being produced by the lower panel.
The ONLY member in significant tension in this structure is the flying wire (the red shows what happens if the flying wire breaks), and in most biplanes with center sections and single bolt jointed structures at the fuselage attachments there are no bending loads at all, except airloads pushing up on the spar beam along its length.


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Why do some fighter jets have double wheels in the front gear whereas some other aircraft have a single wheel?
The Typhoon has a single wheel in the front gear, but the Rafale or Gripen have double wheels in the front gear. What is the reason for this difference?

There are a lot of reasons for doing one over the other, from design trade offs, wheel well geometries and available space in the airframe for the gear, castering and steering mechanisms, weight savings vs service life, runway loading requirements, etc.  In the case of the Rafale-N, the nose landing gear is designed to interface with the catapult shuttle on the ship, so a single nosewheel would not be an option here.

I think we need a question about all the different ways used over the history of aviation to connect airplanes to catapults aboard carriers... if there isn't one already.


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What is this board given to a pilot in World War II?
What is this board given to a pilot in World War II?

Obviously, it's an old iPhone!

The photo in the question is a screenshot from the movie "Midway"(2019).
The board is a Mark 3A Plotting Board used for navigation by pilots flying from U.S. Navy aircraft carriers during World War II.  It can be seen carried by various SBD Dauntless pilots in this scene from the movie "Midway" (2019) from 0:24 to 1:07.
Photo credit: National Air and Space Museum, Smithsonian Institution

Plotting Board Instructions

Photo credit: National Air and Space Museum, Smithsonian Institution

How would you quickly get it out of the way?

It is mounted on a sliding shelfd and can be slid out of the way.

What's the device mounted to the windshield?

Gun sight or bomb sight.

But why did it have to be brought into the cockpit before each take-off rather than just being there? Was it somehow mission-specific?

The board also serves as a map holder and different maps would be attached for each area of operation or mission.


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The NASA X-57 uses 12 electric motors for takeoff. Does it need 12 control levers or just one?
If the NASA X-57 uses 12 motors for takeoff, does it need 12 controllers (like throttle levers), or just one?
The traditional way of engine throttle design was 1 throttle per engine, but what if you have 12 engines? Twelve levers would be unmanageable, pushing a quadrant of 12 levers forward on takeoff.
I suppose that on initial setup, you would need a small dial for each prop to sync rpms. More sophisticated electronics could have a feedback loop to do this automatically.
So... what about just 1 controller, and 2 or 3 backup controllers for redundancy as they are electronic and could easily be blown. I'm thinking of a 3 or 4 position selector switch to choose the active throttle, sort of like choosing between magneto 1 and magneto 2 in a typical GA aircraft.
Yes, you could wire the motors up in banks of 4 and have a quadrant of 3 levers, but my specific question is:
Would having just one active controllers for 12 motors be acceptable?

Setting up a FADEC system controlling all of the engines would be simpler than adjusting twelve knobs every flight. After all, engines wear at different rates. Your performance will be slightly different for each flight.   As far as redundancy, having one throttle per pilot where each controlled all twelve engines would create the desired redundancy. You could even have a primary and secondary FADEC system. Otherwise, a knob, a couple of buttons, or a GUI slider on your glass panel would suffice for backups and save space in the cockpit.

From what I understand, the whole point of the "Leading Edge Asynchronous Propeller Technology" setup is that the motors are not synced and that instead each motor constantly adjusts its speed to shape the airflow over the wing into the desired pattern regardless of gusts and crosswinds. So, really, there would be no throttle at all, instead there is a "virtual throttle" where the pilot indicates the desired performance, and the computer figures out how many motors it needs at which power to most efficiently achieve that performance.

The need for throttles is to provide a linkage to the carburetor/throttle body in the age when everything was mechanical. The throttle in older airplanes directly controlled the air-fuel mixture allowed into the intake manifolds. If electric motors were used, the throttles would have been attached to mechanical rheostats.
In the age of computer controlled engines/motors, this direct linkage is not necessary. You would essentially just tell the computer how fast you would like to go. The system will take care of the rest.
In most cases, the throttle is retained in the cockpit to provide the human pilot with a familiar interface with the computer. Honestly, in an airplane, where throttle settings and changes are more stable and predictable, a knob or a couple of buttons would suffice. FADEC controlled engines/motors are pretty standard now. The method of inputting your desired controls in a fly-by-wire system is arbitrary. It is just a matter of comfort level with the controls. We might even get to the point of a true GUI cockpit one day.
In the case of a multi-engine electric airplane, you would want at least two controls to account for differential thrust when desired. This is not something that you would absolutely need. But, it would be convenient.
Some very complicated aircraft like the V-22 Osprey rely on the pilot not having direct control over the engine-prop systems. It’s absolutely necessary for a computer system to coordinate the two engines during VTOL operation. The same problem would present itself in a quad-copter large enough to carry human passengers.
As an aside, I vividly remember the first throttle-by-wire car our family ever owned. It was a Hyundai Santa Fe. Though, not difficult to drive. The throttle response took some getting used to. Also, modern electric cars with multiple motors have only one throttle.

"We might even get to the point of a true GUI cockpit one day." and heaven help you when you're trying to change throttle settings on a touch screen or with a mouse when you're one engine out on short final in rough weather! The fine control necessary for using devices like that can be difficult enough on a stable desktop. Also, moving your hand back and forth between different physical controls in different places with tactile feedback of grabbing an object that feels like the right object is far different than touching the right place on an a completely flat screen.

Several questions here about the likelihood of touch screens ever making it into cockpits (other than the iPad for EFB), so not going to get into a protracted discussion here. Just wanted to throw out the thought.

@FreeMan - You are right about that. It can be difficult to make inputs into your touchscreen during turbulence in a GA aircraft as it stands today. Anchoring your thumb on the ledge of the screen does help. Having backup knobs and softkeys like the ones currently in use on Garmin, Dynon, and Avidyne units helps even more. Then, there is the full keyboard of a Cirrus Perspective unit. I honestly don’t think that a full GUI system is beyond the realm of possibilities. Is it probable in our lifetimes [shoulder-shrug]? Probably about as likely as the aforementioned quad-copter.

@FreeMan - “ Maybe you just want to fly the plane yourself. Well good luck pressing Take Off, then Autopilot, then Land!”    - Capt. Carol Burnett

@FreeMan - PS. Check out Garmin’s TXi, and other touch screen displays. https://explore.garmin.com/en-US/txi/

@FreeMan - Many of the ones I’ve seen have either a thumb ledge at the bottom of the screen and/or redundant tactile controls. It seems to work just fine for the Phenom pilots I know. I have not had that much difficulty with the one Garmin TXi unit I’ve used in relatively smooth air in a C172. Shooting a practice, non-coupled IAP in a very light SportCruiser with a Dynon Skyview during a 15 knot sustained plus 10 gusts was challenging because of the plane and pilot, not the avionics. It would have been easier to fly the coupled autopilot.

what about just 1 throttle, and 2 or 3 backup throttles for redundancy as they are electronic and could easily be blown

You wouldn't need separate levers for that: just have one lever driving two or more throttle position sensors.
You might want separate levers for the left-side and right-side engines, so you can use differential throttle control to steer the aircraft in an emergency.

For a system with so many electric motors, there is an interface that converts what the pilot wants to what those motors need to accomplish to deliver it. Something similar is also seen in some cars' suspensions and aerodynamics (working in tandem to keep the car under control when an inexperienced driver pushes it beyond his or her skills), and even throttle control.
With that said, an issue with that is what I call the "Eclipse Problem" after the throttle issues caused by the FADEC in the Eclipse 500. Main issue is not the FADEC by itself but that the Eclipse has no backup throttle control as it was designed originally to be but a testing platform for the engines. And then someone bought the design and made a general aviation plane out of it.


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Is the CV-22 / MV-22 Osprey classified as a helicopter, plane or both?

Is the CV-22 / MV-22 Osprey classified as a helicopter, plane or both?

If the President had to hitch a ride on one, would it become Airforce One (CV-22) or Marine One (MV-22)?

They are classified as Tiltrotors, which is its own category for all the obvious reasons.
If the President were flying in a Marine Osprey, the callsign would be Marine One. If he were in an Air Force Osprey, it'd be Air Force One.

Under the FAA, a CV/MV-22 Osprey is considered to be a powered lift aircraft. Under 14 CFR §1.1:

Powered-lift means a heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low speed flight that depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on nonrotating airfoil(s) for lift during horizontal flight.

That designation would depend on the branch of service operating the aircraft.  A USAF CV-22 carrying the President would have the call sign Air Force One during the operation.  A USMC MV-22 carrying the President would have the call sign Marine One.


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Why does NASA say $v^2$ is velocity squared rather than speed squared?
Lift and drag forces scale as the square of the relative flow speed, $v$.  But speed is the magnitude of the velocity vector; that is, speed is the square root of the sum of the squared velocity vector components, by definition.
Why does NASA say that $v^2$ is velocity squared, rather than speed squared?
In aerodynamics, is the sum of squared velocity vector components referred to as velocity squared?

Where does NASA say that?

@CamilleGoudeseune for instance here, https://www.grc.nasa.gov/www/k-12/airplane/lifteq.html

Cf. the generalized use of airspeed instead of air velocity. Yes it is sometimes found on Nasa site, e.g. "The air velocity is the relative speed between the kite and the air. When the kite is held fixed by the control line, the relative air velocity is the wind speed".

"Airspeed" (and "ground speed") are the standard terms used in aviation. Every plane has an airspeed indicator in the cockpit. No plane has an "air velocity indicator".

Don't know about where you saw this, but the expression is Velocity squared because it is normally used in engineering or mathematical expressions to generate other values which are also vectors, not scalars (they have a direction, and a magnitude, as in Acceleration = rate of change of Velocity. Acceleration is a vector, so it is the rate of change of the vector, not just of the magnitude of the vector (speed).  Doing the Math, an object on a circular path experiences an acceleration from it's change in the direction of it's velocity, even though the speed is constant.

You really need to include a specific quote or a specific citation in the question itself.  I'm sure that if you looked through the vast array of material NASA has published over the decades, you could find some instances of NASA "saying" that v represents "airspeed", or "speed".

Also, instead of phrasing your question "why does NASA say" (who knows why anyone does anything), it might perhaps be better to phrase it more like "would it be technically more correct to say that v represents airspeed rather than velocity in this equation"-- or something like that--  but maybe that's not the question that you are most interested in having answered--

@alephzero -- "Every plane has an airspeed indicator in the cockpit. No plane has an "air velocity indicator""-- what, your plane doesn't have one of these?  https://pixfeeds.com/images/math/measuring-instruments/1280-539960480-anemometer-wind-speed-measurement-instrument.jpg

@quietflyer I was going to mention a rate of climb indicator.

I don't entirely understand what your concern is. The square of a vector is generally taken to mean the dot product of the vector with itself, and for velocity and speed v dot v = |v|^2 = speed^2. They're the same thing, just different names

@llama, I think your comment clears it up for me, thinking of the squared velocity as the dot product of the velocity vector with itself -- which gives speed squared.  "Squaring" a vector is not something I hear of often in mathematics -- dot product or inner product is what I'm used to.  Thanks,

Because generally speaking scientists don't use the term "speed". It is always velocity

@slebetman: Scientists use speed where it's appropriate, e.g. light speed, i.e. when the direction is irrelevant. Velocity is a compound attribute, there is no unit of velocity.

yeah it's definitely ambiguous/improper notation, and could just as well mean squaring each component, but if it is used that's what it means

The proper technical term for the quantity is velocity, and it is a vector quantity. Speed is a colloquial name. If physicists use it, they only use it for the magnitude of velocity, but most of the time there is absolutely no reason to use it because the quantity is a vector one and acts as a vector one in all equations.
Now square of a vector quantity is the same as square of its magnitude, basically by definition, because magnitude is the square root of the square (in Euclidean vector spaces), so it does not really matter.
So given the magnitude is not actually use anywhere, why would they suddenly use the term speed if they don't use it anywhere else?

re "given the magnitude is not actually use anywhere" -- I would suggest that the equation that appears in the link referenced in a comment by the asker of the question, https://www.grc.nasa.gov/www/k-12/airplane/lifteq.html , only works if you treat V and L as scalars, not vectors.  If you treat V as a vector, don't you end up with L as a vector too, and pointing in the wrong direction, namely parallel to V?

@quietflyer It also works if you treat CI as a non-scalar, say a rotation and scaling matrix.

"Speed is a colloquial name." Pretty sure it's the technical name for a scalar value.

@quietflyer, that equation indeed only calculates magnitude of lift, but it still works if you treat velocity as vector, because square of vector is a scalar anyway—and magnitude is square root of that square, so you simplify square of square root of square to just square by treating is as vector, which is the point here.

@JanHudec "squaring" the velocity vector to get speed squared makes sense when thinking of squaring as taking the dot product of the velocity with itself.  Llama's comment above clears it up for me. Thanks,

@user59327 Squaring vectors in Euclidean spaces (that physics uses to model space) is indeed defined as taking dot product with itself. And magnitude as square root of that dot product with itself.

Because velocity (being a vector) is a little more precise than pure (scalar) speed - so the angle between the object (aircraft, airfoil, ...) and the incoming air is also taken into consideration.

This doesn't sound right; you're talking about the angle of attack, which is accounted for in the lift and drag forces already, e.g. in the coefficient of lift / drag.  But scaling as speed squared is different from scaling as velocity squared.

@user59327 speed squared and velocity squared is the same thing, because speed is a square root of velocity squared, by definition!

I would suggest that the equation that appears in the link referenced in a comment by the asker of the question, grc.nasa.gov/www/k-12/airplane/lifteq.html , only works if you treat V and L as scalars, not vectors. If you treat V as a vector, don't you end up with L as a vector too, and pointing in the wrong direction, namely parallel to V?

@quietflyer no, it's a vector quantity: https://en.wikipedia.org/wiki/Lift_(force)

It’s just because it is derived from a larger more complete theory of aerodynamic forces where it’s necessary to make the distinction. In this equation the coefficient is simplified but a more complete calculation of the coefficient uses the directional components of velocity.
https://www.grc.nasa.gov/www/k-12/airplane/eulereqs.html

TL:DR
Relative flow over a lifting body has to be described as a vector because Lift and Drag are defined wrt the relative flow vector, and Angle of Attack is a parameter.

Is the sum of squared velocity vector components referred to as velocity squared, in aerodynamics?

Yes. Velocity is used to refer to the vector as well as to it's scalar magnitude in scalar formula. This isn't sloppy, it's compact, and more importantly, it maintains the connection to the underlying vector domain.
$$ =========== $$
The squaring-something operator actually represents a large number of different algorithms that are specific to the thing being squared.
Sometimes, there is more than one way to square a particular thing. For instance, a (scalar) distance squared might represent an area, or it might not. But the same symbol is used for the operation, and the associated units are often rendered identically. Sometimes it is important to keep track of the distinction and sometimes it isn't. An area has properties that the square of a distance does not have. Just because two things have the same units doesn't mean they have the same set of properties.
The same applies to velocities and speeds. Both speed and velocity have the same units, but they have different properties. Both can be squared, but the procedures are different.
So to get square of the speed of the relative flow, you take the magnitude of the relative flow, then square it. To get the square of the velocity of the relative flow, you square the velocity (a vector operation), and then take the magnitude if you want a scalar.
It's not luck that $ scalar square ( magnitude (vector))$ and $ magnitude (vector square ( vector)$ have the same numeric value and units, The operations were designed to work like that.
But do they have the same properties? Not really. When talking about velocity as a scalar, we are reminded that the domain is a vector domain and that different vectors can have the same magnitude.
Which brings us to a problem, different relative flow vectors with the same magnitude do not have the same Lift and Drag as each other. The only way the formula is valid is if the angle between the Lift and inflow remains fixed and the angle between the drag and the inflow remain fixed, which they do by definition. But all these have to remain fixed with respect to the lifting body as well. So the statement is only valid for constant angles of attack. To suddenly introduce a vector constraint on an entirely scalar chain of math is plain bad form.


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What is the communication protocol for the side-stick in the F-16 or F/A-18?
The side-stick in the F-16 or F/A-18 communicates with the flight control computer to transmit the pilot inputs. This transmission uses a protocol. Which protocol?

There is no side stick on an F/A-18 Hornet.

No side stick as mentioned and they are analogue signals in at least the F/A-18 A B C D. Discrete signals for the buttons. Protocol to me would imply some sort of digital databus tech such as 1553 or ARINC 629, and that isn't how this is done. It uses LVDTs.


text:
Where is the control loop of the actuator closed?
When electro-hydraulic servo-actuators are used for the actuation of control surfaces in aircraft, linear variable differential transformers (LVDTs) are used for position or velocity feedback. There is a control loop that compares the actual position with the commanded position.

Where is the loop closed? In the flight control computer or in a module near the actuator? By this, I mean where this comparison is taken place?
What are the pros and cons of each of these methods and which one is usually used in modern aircraft?

Different aircraft and manufacturers take very different approaches.
1. Boeing 777
The B777 has three Primary Flight Computers (PFC) that are responsible for flight control laws computation and four Actuator Control Units (ACE) that are responsible for the closed-loop control of their responsible flight control surfaces.
The ACE is primarily an analog device, including the actuator control portion. Note that analog here refers to any control system that is not a general-purpose computer. Analog devices have a finite number of possible failure modes. For this reason, it is enticing from a common-mode/generic failure perspective that bedevils digital computers. The main disadvantage is that it is less flexible and powerful than a digital computer.

Ref: http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf
2. Airbus A320
The A320 closes the actuator loop directly on the five flight control computers: 2 Elevator and Aileron Computers (ELAC) and 3 Spoiler Elevator Computers (SEC), plus 2 Flight Augmentation Computers (FAC) for rudder controls.
In order to combat against common-mode failures, SEC, ELAC and FAC are designed to have completely different architectures by different design teams. Software is also designed to ensure dissimilarity across the command and monitoring channel of each computer.
An in-depth discussion on common-mode failures can be found in this paper, which cites software, and in extension, general-purpose computing, as a major source of common mode failures.

Ref: http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_12.pdf
3. Airbus/Bombardier A220
The A220 accomplishes the closed-loop servo control in Remote Electronic Units (REU) located fairly close to the control surfaces and their actuators. For this reason, a total of 10 REUs are located across the aircraft.
The Primary Flight Control Computers (PFCC) normally compute the flight control laws. Since REU is a digital computer, in the scenario where all PFCCs fail (for example, a generic failure) and if sufficient number of REUs remain operational, the REUs can take up the slack to run Direct Mode control laws.
The primary advantage of this setup is the reduced wiring weight compared to centrally located servo-loop controllers, as well as higher failure tolerance in the case of single servo-loop failures.
The primary disadvantage, once again, is the common-mode failure across the REUs. Due to the high number of REUs, it becomes prohibitively expensive to design sufficient dissimilarities into them. For this reason, a third backup control loop, in the form of an analog Alternate Flight Control Unit (AFCU) is needed to cater for this remote failure mode.

Ref: https://quizlet.com/421493192/a220-lod-ch9-flight-controls-flash-cards/

Thanks for the response. Suppose that a PID control is used to adjust the surface to a specific angle. So the true angle is subtracted from the desired angle and the error is sent to the PID and the loop is closed. You are saying that this subtraction and PID module are located in flight control computer for A 320 and for Boeing 777 and A 220 these two functions are located in ACE and REU. Is that true?  Besides that do you have any similar information for F/A-18, F-16 or F-22?

I am a little confused on what you mean by flight control law computation. By flight control law computation you mean the angle that is computed for each surface or the signal that should be applied to the actuator?

@shooshool The end point of flight control laws are typically surface commands. The servo-control loop then closes the command with the actuators at much higher bandwidth. It's a cascade control architecture.

Do you have any reference that suggests common mode failures occur more often in digital computers compared to analog modules?

@shooshool I've added a link and made a note on "analog". In my understanding, FPGA based finite-state logic system can be regarded as "analog" for the authorities, even though it's technically not analog.


text:
Can you change airways when they cross without a named fix?
There are a few victor airways that cross without a named fix, just two solid lines drawn over each other on the IFR low charts. Can these be used as intersections for changing airways in IFR route planning? Assuming you are above MEA for both, prior to the crossing. They aren't very common but I happen to have one very near my usual airport that along with a poor DP makes planning for certain directions rather tedious.
Examples

the crossing of V55 and V170 just north of JMS VOR on chart enroute L-14 North Dakota;
V495 and V23 with V187 over Tacoma(this one may be due to SEA class B) ;
V183 NE of RZS near Santa Barbra, could you plan to turn north from V183 onto V485 or V107?

Interesting question.  Bullet #2 is in my neck of the woods so I checked it out.  (I don’t see V495 coming into play though...) I think it would be a problem, mainly with planning and filing because ForeFlight wouldn’t allow or show the transition when I tried just now.  But, perhaps you could get around it creatively?  E.i.  File:  “SEA, V23, (request radar vectors) V187, RADDY...”  Or, pull up a lat/long for the point and insert into flight plan?

I may need to modify my question, my intent was focused on fully by the book flight plans, usable with lost comms or lost radar and using traditional nav. (excluding RNAV.) More of a pure theory question.

Given all those conditions I think the answer is probably no.  But truly the only limiting factor is that the intersection doesn't have a name.  Because it is an intersection...

Taking a look at this again out of boredom, and at least for the Tacoma example I can't imagine why either you would want to transition at this non-intersection, or why ATC ought to let you.  It is a 90 degree turn, in class B airspace, why not simply cut the corner and go ALDER to RADDY, or vice versa?  Do the other examples have a compelling reason why someone might want to do this?

Yes one can fly one airway to intercept another. Many reasons exist as to why not all intersecting airways have an intersection, the simplest being, it wasn't necessary.
I will suspect one may encounter difficulties in executing your desire to change airways as stated. Such would be the inability to file a flight plan with dispatch or that flight planning software can't/won't accept this unconventional navigation method and the most illogical, if one could accomplish the later, I would image would be ATC. Why? Because the controller has never encountered this situation, before and doesn't want to explain their actions should something go afoul.
For years ATC would issue preferential routes to aircraft departing KPIA to KORD as such; PIA PIA030R V10 PLANO ORD. Aircraft would depart KPIA on the tower assigned departure heading then be vectored to join the PIA 030-radial to intercept Victor-10 northeast of BDF VOR onward to KORD. Then computers became more involved along with flight management systems and pilots were no longer capable to navigate this route because computers couldn't calculate the routing or how to intercept one radial (airway) from another without a set point (intersection) along the second radial , something pilots had done since the 1950's.

the IBM 9020 (and subsequent Center computers) will join airways if entered in the format "J17..J94", as long as the airways actually cross (cross point is adapted on both airways as a computer fix). A radial to join an airway has to have an adapted point on the airway. The adaptation usually only joins airways of like types, so, J-route to J-route, but not Victor airway to J-route.

@atc_ceedee Interesting. So then is it not possible for a pair of airways to intersect in more than one place?

No, it's entirely possible. V589 and V85 from MBW to DDY (Wyoming), for instance, take different routes to the same points. This happens a lot in mountainous areas, where some aircraft can't use the more direct, higher MEA airway, and opt for the other airway that lets them stay lower. Generally, in my airspace, this is an onboard oxygen issue, but there could be other reasons.

Another example is V611 and V81, which both pass over BRF (Black Forest, Colorado Springs) northbound, V81 to the northwest, V611 to the northeast. They intersect again at CYS, with V81 going northeast, and V611 going northwest. The crossing points are VORs, so these should be filed if going from one airway to another.
At KGCC, we teach trainees to use V524 from the south as an "inbound" airway, because it has a straight-in ILS from TOOKE. We use V298..V247 for southbound departures with degree divergence, to keep the airport running smoothly.


text:
Why do so many airports have the same CTAF frequency?
In the United States, many non-towered airports have CTAF frequencies (often shared with UNICOM as well) that tend to overlap with various other airports within a 50-100 nm radius, at least in more populated areas and states. I personally do not find this practical, as it congests the frequency. Also, for every radio call, you have to carefully listen to each call in order to know with respect to which airport the aircraft is calling.
Of course, there are other hints such as runway numbers, but the congestion is still not ideal because it increases the workload, makes it harder for you to say something, and increases the probability of radio collisions. I also noticed that in my area (Northeast USA), the CTAF frequencies are typically multiples of 100 kHz, even though we have a 25 kHz channel spacing system in place.
My question is essentially, why is there such a huge degree of overlap? Are there really no unused frequencies remaining to assign to all these airports? And if the answer is no, how about transitioning to 8.33 kHz channel spacing? Does there exist any plan to have better separation of frequencies at any point in the future? I am not yet an experienced pilot, but I do think that the ability to listen to only your airport and not 2 to 4 additional ones could significantly reduce the workload associated with non-towered airport operations and radio communications.

The CTAF frequencies 122.7, 122.8 and 122.9 are called MULTICOM because they are deliberately not specific to one airport. These frequencies are holdovers from the days of 100kHz radios, when we didn’t have frequencies to spare. They’re also easy to remember and recognize.
Today, MULTICOM is still used for low-traffic airports, so there shouldn’t be too much congestion, and the same frequency is often assigned to clusters of airports in close proximity so that you’re aware of other traffic in the general area—or in practice areas between those airports.
With the advent of 25kHz radios, it became possible to move every airport with UNICOM service to a discrete frequency, and for the most part, this has been done. If your airport hasn’t changed, you can discuss that with the airport manager and/or FBO, and they can contact the FAA to get a discrete frequency if warranted. Note they may have deliberately not done that for safety reasons due to proximity with MULTICOM airports.
Also, UNICOM frequencies still do get reused, just like any other “discrete” frequency. Reuse distance is based on the expected altitude of aircraft using it, so you will occasionally hear aircraft far away if one or both of you is higher than expected. For instance, you may hear distant traffic at 3k AGL that goes quiet as you descend and enter the pattern at 1k AGL.  That is normal.

For clarification, in NJ, even at pattern altitude, I can typically hear multiple other airports. The majority of airports indeed use 122.7, 122.8, 122.9, or 123.0.

@aviator On a UNICOM or MULTICOM frequency?

CTAF/UNICOM, there is rarely a MULTICOM (at least judging from the chart supplements)

"MULTICOM because they are deliberately not specific to one airport", yes, the specificity of multicom is this is a service which frequency is reserved but not licensed to any ground station in particular (air-air links). Unicom is a ground station for ground-air links, with a licensed frequency.


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What are the components of a crew pairing?
I need to do some modeling for an FRMS (Fatigue Risk Management System). By crew pairing I mean a sequence of flights that starts and ends at crew base. From my understanding, crew pairings are made of:

Duty periods, which include flight duty periods (where the crew actually flies).
Between duty periods, rest periods (overnight rest for example), or
You can also have deadheading before starting another duty period.
So basically when the crew finishes one duty period, he/she either has a rest period or some deadheading before starting another duty period.
Is there anything else that should be included in the pairing?

Deadheading is considered duty, so it is part of the duty period, not part of the rest period.
Each duty period is composed of several parts:

a report time, defined in a specified timezone (perhaps GMT, perhaps local, perhaps "home" time of some sort)
time from the report until the first departure, which is typically standardized although it may be variable (45 minutes domestic, 60 minutes international, 75 minutes international overwater, for example)
a series of flight legs, each with its start & stop times, duration, start and end airports, and deadhead status
time between each leg
time from last arrival until release from duty, which accounts for the time on duty required to post-flight the aircraft, clear customs, etc. This is typically standardized in some fashion; xx minutes if clearing customs is not required, yy minutes if it is

For each duty period that does not end back at the base, there will be the intervening crew rest period, or "overnight", which consists of the start and end times of the rest period, its duration, and in practice a specified hotel where the crew will stay.
The pairing itself will typically be published with some summary values, such as total flight time during the pairing, total duty time in the pairing, etc.  If the crew contract specifies "overrides" for pay purposes, these will generally be published as well.  As an example, let's say that the contract specifies pay credit of 1.2 minutes paid for every minute of block time between the hours of 2300 thru 0500, and a particular flight operates from 2200 to 0100.  That is 3 hours of block time, but the last two hours pay at 1.2x, so the pay credit for that leg would be 3.4 hours, or 3:24 of pay even though it's only 3:00 of block. Other contractual rules may specify a minimum pay per duty period, such as 4:00 pay, even if less flying than that takes place. The pairing summary would generally include the total pay credit for the trip; each day's summary might include how that day's pay works out.
Part 117:
The rules of FAR Part 117 specify legal requirements for duty and rest periods; they include the definition of a "Flight Duty Period", which begins at the required report time, and ends at the block-in of the last operating leg. Thus, FDP ends even when a crew remains on duty.  If the last leg of the day is a deadhead, then the FDP might be six hours but the duty day might be longer -- say an hour's ground time, and hour's flight, and 30 minutes to clear customs & leave the airport. When the crew is off duty, then the rest period begins.
Depending on the operation and the contract, the pairing details may align closely with Part 117, or there may be enough additional considerations that what's published on the pairing may go way beyond the Part 117 definitions.  The FDP as defined in Part 117 (along with the other limitations in that regulation) are still tracked, although in some cases contractual constraints may be more limiting or require more bookkeeping (i.e. accounting for pay rules and contractually required rest periods).
FRMS:
When you're looking at things from the perspective of a Fatigue Risk Management System (FRMS), there are other factors to consider.  Not everything that is legal is necessarily safe, and just because some sequence of duty periods is legal doesn't mean it won't be really fatiguing.  For example, a sequence of 11-12 hour duty days flying in the same time zone may be fine, while the same sequence of 11-12 hour days crossing 5-6 time zones will shortly have the crew operating on the "back-side of the clock" in local time, which has its issues.  Worse, if each 11-hour day is followed by a 16 hour overnight, the crew could be waking up later & later (relative the their body-clock time), but operating earlier and earlier in the day (as measured by daylight/nighttime looking out the window) -- which is a serious setup for "not knowing what time it is".  An FRMS would probably need to consider how well or poorly a pairing or set of pairings works when considered against circadian rhythms and similar factors.  This would involve tracking data beyond what is published for the typical crew trip-sheet.
At a minimum, you might want to see flights and days and rest periods considered in both a constant (say, "home" time) reference and also a "local time" reference -- even if what is published to the crews is all stated in Zulu time.
Block Time:
The question was asked in comments, when does Block Time begin & end? When the schedule is being built, Block Time corresponds to the scheduled departure and arrival times of the flight. The book definition of block time is that it begins when the aircraft begins forward movement under its own power for the purpose of flight, and ends when it comes to rest with no further movement intended. In practice, block time is almost always recorded by ACARS, which uses "all doors closed + brakes released" -- i.e. the start of the pushback operation -- as the block-out time, and "entry door open" (i.e. pulled into the gate and door is opened to let people out) as the block-in time.
This practice slightly over-states the block time as compared to the book definition, since one can release the brakes, then begin the pushback (under tug power, not aircraft power, and not "forward movement"), start engines, and so forth, and several minutes may elapse before the aircraft actually moves forward.  And, if you arrive at the gate but the jetbridge isn't quickly pulled up to the aircraft, you may be stopped for some length of time before the ACARS records the "in" event.  These distinctions are evidently acceptable to the FAA, since this seems to be a fairly universal practice to use ACARS times.
Typically, the reporting from the aircraft back to Dispatch or headquarters will have "out" (i.e. out from the gate, block time began), "off" (i.e. off the ground, flight time began), "on" (i.e. on the ground, landed), and "in" times. The aircraft logs are mainly interested in flight time, from "off" to "on", while crew recordkeeping is pretty much exclusively interested in block time, from "out" to "in". When FAR 117 refers to flight times, it is actually referring to block time "out" to "in" and is unconcerned with actual takeoff & landing times. One explanation being, the crew is as much responsible for the operation of the aircraft while taxiing around JFK on the ground, and needs to be alert in order to avoid mistakes (entering a runway without clearance, for instance), as much then as when they're in the air. So all "operating" time counts the same, on the ground and in flight.

Ralph J Many thanks for your very thorough answer.

One last question, when does block time starts/ends?

@BigONotation  Glad to help. Edited answer for block time into the answer itself -- a bit long for a comment.

Ralph thanks a lot this is very helpful indeed!

@ymb1 Good edit, thanks

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text:
Reporting to ATC when losing visual to traffic?
I'm a PPL student and I recently flew to a class C airport. One of my first flights into class C, actually. Once I contacted the tower and stated my intentions, I was asked to report if I had another C172 in sight at my 12 o'clock, 2 miles. I did, and reported traffic in sight. Tower then gave me approach clearance to the active runway as number 2.
However, it was an overcast day and I quickly lost visual with the traffic. Tower didn't specifically say "follow", only that I'm number 2 in the approach (was not yet in the pattern).
Was I supposed to inform ATC that I had lost the traffic?

Are you sure the tower gave you an approach clearance?

The short answer is yes, if you have called traffic in sight then ATC has an expectation that you will be able to maintain sight and provide your own separation.  You need to notify them if you are no longer able to do so so that they can resume providing separation service.
However, you are not expected lock eyeballs on the target and maintain sight 100% of the time.  You also need to continue to scan your instruments and keep a good lookout for other traffic.  It is understandable that you might lose sight periodically before you see them again.  The time spent trying to re-acquire before you confess on the radio depends on the situation, and this is why situational awareness is so important.  What you actually say, (other than a simple "lost sight") may also depend on the situation.
Let me offer a couple examples:
SCENARIO 1:  In the example you provided, both aircraft are of the same type and are presumably both on approach to the same runway.  2 miles is a full minute of separation at 120 knots, so as long as they are not excessively slow and you are not excessively fast you should be able to maintain plenty of spacing.  Keeping sight of a tail-on aspect, lost in the ground clutter with no track crossing angle to cause your eye to catch the motion, can be very challenging.   Unless tower directs the first aircraft to perform a 360 for spacing there is no reason to anticipate a conflict.  You still need to ‘fess up on the radio, but rather than compromising the rest of your scan by implying that you will keep looking hard for this one airplane, I might attempt to shed responsibility by saying something like “Tower, N123 unable to maintain sight of traffic, let me know if it becomes a factor” and carry on with my approach.  They are generally understanding, and if it isn't urgent you will likely hear a friendly "Roger, traffic is on short final now, no factor."
SCENARIO 2:  On the other hand, let’s say you are on that same straight in approach, maybe dragging in a little bit low in a high wing 172, and tower calls #1 traffic as a Piper on downwind.  You initially see the traffic on downwind, but subsequently lose sight.  When that low wing Piper starts their base turn, (and turn to final) they will likely lose sight of you behind the wing, and if you don’t have them in sight the safety risk is far, far greater than in the first scenario.  In the second case the hair on the back of your neck should go up, and you need to speak up early and often until you are assured that there will be no conflict.  I have been criticized for offering examples of radio communications that aren’t strictly by the book, but in any non-standard situation the most important thing is to get your point across.  And be directive if you need something.  Examples of what I might say include:  “Tower, N123 lost sight, say position of Piper”.  “Tower, N123, is the Piper on base leg yet?  I've lost contact.”  “Tower, N123, call my traffic please.” "Tower N123, where's my interval?!".
Be assertive, speak clearly, but most importantly don’t hit another airplane worrying whether you should speak up or fumbling over proper phraseology when simple plain language will inform ATC of your situation and let them give you the help you need.

"tower, I can't see them, can you call my base?"

@tedder42, if you are in the Piper, absolutely!

Thanks, that was a nice explanation and the examples were great!

Yes, it's good to inform them because if you don't, they assume you know exactly where it is and you are responsible for keeping your distance.
The appropriate call to ATC would be:

Tower, 123AB negative contact

You could add a little more on it, like "traffic no longer in sight", but the above is technically the magic words for ATC.

Eh, that's not so great. You are correct that "negative contact" is a fully sufficient response to an initial traffic call, but if you call "in sight" and subsequently lose them then a bare "negative contact" is a bit too terse and might be confusing. We aren't primed for hearing that except in the context of an active calling-traffic conversation. I would prefer "Tower, 3AB now negative contact on the Skyhawk" or something like that.

I thought of this as well @randomhead, I probably would have said the latter suggestion in this answer next time this happens.


text:
What does a pilot mean when he tells ATC that he "has the numbers"?
What does "have the numbers" mean when talking to ATC?

This is from the FAA's Pilot/Controller Glossary:

HAVE NUMBERS − Used by pilots to inform ATC that they have received runway, wind, and altimeter information only.

The AIM has some more information in 4-1-13 and 4-1-18, which comes down to "having 'the numbers' is not the same as having the ATIS". The equivalent controller information is in the ATC Orders 3-9-1, 3-10-1 and 7-1-3. For example:

3−10−1. LANDING INFORMATION
Provide current landing information, as appropriate, to arriving
  aircraft. Landing information contained in the ATIS broadcast may be
  omitted if the pilot states the appropriate ATIS code. Runway, wind,
  and altimeter may be omitted if a pilot uses the phrase “have
  numbers.” Issue landing information by including the following:
NOTE−
Pilot use of “have numbers” does not indicate receipt of the ATIS broadcast
  [...]

In most cases, pilots will just tell ATC that they have the ATIS anyway, as Ron described in his answer. But, there are some towered airports that have no ATIS, only AWOS, e.g. Columbus, Indiana (KBAK). In that case, telling the tower controller that you have the numbers is the only thing you can actually do (other than not tell them anything and let them give you the information, of course).

This typically refers to the altimeter setting and the wind speed/direction. The pilot usually gets this either by listening to other transmissions from the tower or by listening to ATIS prior to contacting the tower.
Usually when I make the first call to approach or tower, I've already listened to ATIS and have the latest report. So my first call will go something like this:

Green Bay Approach, Cardinal N12345 inbound to land with Kilo

ATIS is usually hourly (unless there is a major change) so tower will typically tell whoever is calling what the most up-to-date wind/altimeter is when they first contact. Saying "I have the numbers" just tells tower that they don't have to repeat it, although it really doesn't save time.
It should be noted that according to the AIM 4-1-8, a pilot saying "have the numbers" does not indicate to the controller that they have listened to the latest ATIS. ATIS information includes airport advisories so controllers will not assume you listened to ATIS if you say "I have the numbers".
So, if you listened to ATIS say "with [insert ATIS letter here]". If you haven't listened to ATIS or you are arriving at an airport with AWOS/other weather broadcasts, you can say "I have the numbers".
Also, you should never use "I have the numbers" when flying IFR.


text:
Can helicopter or aircraft intro flight hours count towards a private pilot license?
I'm trying to figure out whether I'd prefer to fly helicopters or fixed-wing aircraft and am taking a few intro flights where the other person lets me fly for a little bit.
Can these hours count towards my helicopter / aircraft private pilot certificate? If yes, do I need a logbook or what kind of proof do I need?

Which country or regulations are you asking about?

Get your fixed wing PPL first, you don't want to be paying $300+/hour to learn how to use the radios and airspace, once you get your PPL in fixed wing, the transition time is shorter for helicopters.

Ron, I'm not very well-versed with how licensing works. In the aircraft world, do people get a private pilot's license with an endorsement for only fixed-wing, and then you can get an endorsement for rotorcraft with additional training (it's actually the same license, just different endorsements)?

Yes, these hours can be counted towards PPL, if you fly with a flight instructor next to you, which is generally the case if you go to a flight school and ask for "introductory flights" anyway.
In my case, they provided me with a tiny logbook for free so I can log my hours.

And if you give them a copy of a birth cert or passport so they can make a TSA endorsement.

Might be worth saying though that while these introductory flights technically count towards the required hours, they do very little in the form of teaching and really aren't that beneficial in gaining your certificate. Most pilots take significantly longer to get the license than the bare minimums, so I wouldn't worry about getting these hours recorded, usually the log book entry is more of a sentimental thing (hey, I started here).

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text:
Why do pilots do walkaround inspections for regularly scheduled commercial flights instead of aircraft mechanics?
If the airline has personnel to maintain the aircraft, wouldn't it be more cost effective to have them inspect the aircraft before flight rather than have the captain or first officer do it?
Is it because whoever will fly the aircraft will probably do a better job of inspecting it because they are putting their own lives at risk?

"Is it because whoever will fly the aircraft will probably do a better job of inspecting it because they are putting their own lives at risk?"  There's a history of accidents with aircraft just out of maintenance.  I've often thought that the mechanic should have to fly on that first flight, just to give them an extra incentive to check for forgotten spanners in the engine bay.

Related:    Do pilots of airliners and jumbos do a walk around preflight check for every flight? https://aviation.stackexchange.com/questions/47024/do-pilots-of-airliners-and-jumbos-do-a-walk-around-preflight-check-for-every-fli

Do the various airlines have mechanics at all the airports they fly to?  Even where they do have maintenance facilities, they usually seem to be a long way from the passenger terminal.  Besides, having mechanics do the inspections takes them away from other work, while the pilots probably aren't doing much while passengers are deplaning & boarding.

@DaveGremlin You don't have to look far to see a tragic disconnect between actors and consequences.  Didn't work in the Soviet state... fear of a lingering death from radiation poisoning did not motivate anyone at Chornobyl. Things work so much better in capitalism, just look at how the USA is hitting top numbers in the fight against COVID...

There's also the matter of having personally experienced the last flight.  Like, (overly simplistic example) the pilot felt a bit of a bump on the left side about an hour into the flight, might want to check if was a bird strike or just turbulence.  Or they thought they saw some warning light flash for a second but then it went away.  The ground crew might not even think to check for that if the pilot didn't report it to them.

@MikeSowsun your answer to the question you just linked to says that at your airline the pilots typically don't do the pre-departure check on widebody aircraft.  I think that deserves to be an answer to this question too.

@Harper-ReinstateMonica: Mistakes will still happen.  If Chernobyl had been remotely operated by the same people with the same policies, who's to say they wouldn't have made a critical mistake (excuse the pun) sooner.  Your argument just proves that motivation to not die doesn't make a safety system foolproof, not that it doesn't help.  It's widely believed that it's a good idea to have the person who's life is on the line involved, e.g. in packing or inspecting their parachute, whether it really helps or not.  If it was someone else, they'd maybe have to live with the guilt of a mistake.

What make you think they don't both make a walkaround inspection? the pilot is the last one to do it, but that does not prevent anyone else (e.g. an aircraft mechanic) to do this inspection before.

Some militaries (US Navy?) traditionally leave the walkround to the crew chief, don't they?

Hmmm, I wonder if timing makes a difference?  I'd expect a pilot to typically do a check closer to departure time than others.  A check an hour before would be more timely than 2+ hours before.

@DaveGremlin Re having the mechanics fly on the first flight after maintenance was done, it would really be great if it was possible, and even better have the mechanics' manager fly with them too, I mean the person who decides mechanics' schedule, priorities, procedures, expected task duration, incentives for the "fastest" workers etc.

Because it's the Captain's plane, for all intents and purposes. This is exactly the same for many other industries - in particularly shipping (as in boats) and haulage (as in trucks).
Similar to planes, ships and boats have captains and they are all fundamentally and legally responsible for everything that happens on that mission. And, likewise, a truck driver is expected to maintain responsibility for their truck - indeed, over here in the UK this is a legal requirement and truck drivers have to do a mandatory walk-around check.
It's also the same for car drivers - you, as a driver, are fundamentally responsible for your car and its safety.
Focussing on aircraft, the pilots probably have the single most holistic view of the aircraft, its mission and the requirements. In addition, the pilot is always going to be there - whether that's Heathrow at 2pm on a Tuesday, or a deserted dirt strip in the outback at 10pm.
The better question might be - what good reason would there be to skip a walk around in one of the most safety-critical industries? At best, for some airports in some circumstances, you can argue somebody else might be able to conduct the walkaround - but it's not guaranteed and it introduces another potential point of failure or miscommunication.

It is the same in the US. The FAA sets the responsibility for determining the aircraft is airworthy on the pilot  in command.   I expect this  rule is pretty much universal.
Ref:
§91.7   Civil aircraft airworthiness.
(a) No person may operate a civil aircraft unless it is in an airworthy condition.
(b) The pilot in command of a civil aircraft is responsible for determining whether that aircraft is in condition for safe flight. The pilot in command shall discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur.

@Gerry: Of course, a pilot also has broad discretion as to what "discontinue the flight" should mean if a craft becomes borderline unairworthy at 40,000'.  Depending upon circumstances, a pilot might actually extend a flight if an airport that's slightly further away than the intended destination has better emergency facilities.

@supercat In that case, I feel like the fact that the FAA allows all rules to be overridden in an emergency makes the point moot :-P

"It's also the same for car drivers" yes, in the UK drivers are obliged to check all your car's lights work before driving https://www.gov.uk/check-vehicle-safe which it's not possible to do without walking round the vehicle, with a £2,500 if you don't. I've never seen anyone actually do this.

@PeteKirkham No, most people don't and the Police are pretty relaxed. The better example is trucks - they have actual walk around checks and the good firms do enforce it. VOSA routinely stop and check trucks, so you'd better have a good excuse ready for any defects!

Ships, not boats

@DwB Both - captains on small boats have plenty of legal responsibility, too. Ask Jack Shepherd!

As @Gerry has posted, in the US, FAA regulations place the responsibility of maintaining the aircraft on the owner/operator. But, the aircraft’s airworthiness is the responsibility of the Pilot in Command. That is the regulatory reason.
The practical reason is one of redundancy and common sense. An aircraft needs to be inspected before every flight. Although, a mechanic will not always be available before every flight to do that, the PIC will.
Also, both the mechanic and the pilot are human. It makes more sense from a safety point of view to have as many different eyes inspect the aircraft for deficiencies. What one person may overlook, another person may catch. The better question is, why in a two or more pilot system, why is each pilot not required to do a walk-around inspection.
When I fly small GA aircraft with another pilot, one of us does a full and detailed preflight inspection. The other pilot does at least an abbreviated preflight inspection on the outside of the aircraft. They then participate in the preflight inspection and run-up in the interior. At the bare minimum, the other pilot has to visually inspect the fuel and oil quantity, flight surfaces, air/cowl inlets, and that all caps, doors, hatches, tie-downs, and chicks are secure. Once inside, they have to check that all control surfaces are free, clear, and correct, and that the brakes work.

When my brothers and I were little boys and our dad wanted to take us for a spin, he had to actively prevent us from doing the visual check with him, as we would happily pull on control surfaces, blow into the pitot tube and so on. Sure, we didn't qualify as the second pilot you mention.

"It makes more sense from a safety point of view to have as many different eyes inspect the aircraft for deficiencies. What one person may overlook, another person may catch" EXCELLENT point!

Trains also go through visual inspection by the drivers before they begin their journey. It's a bit different for large cargo trains and passenger trans with a lot of connected wagons (there  it is a crew task), but up even then the driver is part of it.

The pilots are the only people who have all three of these attributes:

The training and experience to know what is safe.

The legal authority to stop the flight.

A deep personal stake in the outcome of the flight.

That is the safety trifecta.  A mechanic has the training and experience to judge the safety of the plane mechanically, and the authority to stop a plane from flying, but does not have that deep personal stake in the outcome of the flight.  I'm not saying that the mechanic doesn't care, but it's human nature to care more when you could die from your mistakes.  Although the pilot's training does not include the depth of mechanical knowledge that the mechanic has, the pilot is trained to spot the things that can be spotted visually that indicate an airplane is unsafe to fly.  Not all mechanical mistakes can be spotted by the pilot, but many can.  The pilot is looking for things out of place, missing fasteners, leaking fluid, deformation, and anything else that can indicate an aircraft that is not safe.
As an aside, this safety trifecta is why passenger planes piloted remotely or by automation are a terrible idea.  The pilots are removed from the equation, and now the people who are deciding whether the plane flies or not don't stand to die if they get it wrong.

Have to disagree with your first bullet, (particularly in relation to a modern multi-jet) - the pilot knows about 1% of what the mechanic knows about what could bring the plane down.

@MikeBrockington The pilot does know how to spot the problems that it is possible to spot in the preflight inspection.  The rest of the problems, the ones the mechanic is trained for, you need to tear down the airplane to spot.  Even if the pilot had the mechanic's training to do that, it is not practical.

@Mike Brockington, what is your basis, and/or what are your credentials for making such a broad claim? Because I strongly disagree.

I'm not really satisfied by the answers "they walk around because they're required to do so".
Besides the fact that others might have a lower stake in the health of the plane, it's worth noting that over the decades, the culture of the aircraft industry has been forcefully changed, due to crashes happening for dumb reasons.
In an episode ("Check Yourself") of Hidden Brain, there was a particular focus on a crash at a flight show back in the 1940s or 1950s, where they were introducing the B-52 bomber (in service since 1952). Since it was at a flight show, obviously these were top pilots who were doing the demonstration, but anyway, they pitched up too steeply from takeoff and fell out of the sky.
Some people survived the ensuing fire thankfully. It turned out that some sort of flap was locked in place, which was meant to act as a brake on the ground; and had the effect of increasing the pitch. This would typically be unlocked from the outside, but in this new model, it was done on the inside. It seemed to be that unlocking it skipped the mind of everyone and if they had realised in time upon takeoff, they might not have been able to lean over and turn it off anyway.
The naïve solution of "just be more careful" wasn't feasible − these were accomplished pilots; and pilot training could only take so long. I encourage you to listen to the episode, but to summarise, the main thing to come of this investigation was the decision to start using checklists of 6-10 checks − the top few dumb reasons that might cause crashes.
Over the years there were other scandals too, eg Korean Air, which was largely blamed as a cultural issue. Korea has a culture built heavily around respect for different classes − your boss, your elders. This culture is seemingly more extreme than what exists amongst its East-Asian neighbours; and it's built into the language as different forms of address. If they're asking for something or trying to make a point, they beat around the bush because they're terrified of being too blunt.

As the remedy for all the crashes, the pilots were encouraged into the notion that it's OK to speak up if you think something's wrong; and more drastically, they were also told that from now on, they had to speak English, like all the other pilots and airports were typically doing.
English doesn't have the different forms of "you" like so many other languages do; and similarly, trying to express all this indirect desire in English is avoided because it sounds weird − there's an example of a Korean pilot repeatedly expressing that basically he was a fan of weather reports, in regards to his captain blaming the weather reports for the fact that he was unexpectedly flying through clouds, in the wrong place, about to crash into a mountain.
In another industry, it might indeed seem normal to allocate someone else to walk around the plane. If someone else said afterwards though, that they too were going to inspect it, it would probably be perceived as a sleight − "I already checked it. Are you saying that I'm blind?"
If it were as simple as saying "the pilot has the most at stake, so she has to do it", then we could point to seatbelt laws − in eg NSW, Australia, it's the driver who gets punished, if some of the passengers aren't wearing seatbelts.
In this aircraft scenario, the pilot has internalised the fact that he has the most responsibility. He wants to check the aircraft, and it's not going to offend anyone else who was already expected to notice anything.

Just to add another argument to the above questions: having a pilot do a walkaround instead of a maintenance worker includes another point of view. Maintenance crew usually work in teams and assumptions can easily be made.
Imagine a hatch that's not sealed. A maintenance worker might assume some other worker will close it or is still working on it.
A pilot seeing the hatch open will not assume this and see it for what it is; something out of order.

It is part a pilots training to do this. He is the Captain and he/she is responsible for the passengers lives. Tradition and common sense. Especially for small aircraft where no other qualified people are there.


text:
Is data approval by DER/DAR still required under new FAR part 23?
New FAR Part 23 rules have been enacted on August 30th 2017, simplifying certification for small airplanes.
Will the FAA still require:

Designated Engineering Representative (DER) approval for each drawing, and
Designated Airworthiness Representative (DAR) conformity of each part?

This is not required for Light-Sport Aircraft (LSA).

It's not entirely clear how the process will work. The FAA hasn't updated AC 23-17C in response to the rule changes. I also haven't seen any discussion of changes to Production Certificates.
That said, Part 23 is still "AIRWORTHINESS STANDARDS: NORMAL CATEGORY AIRPLANES"
and there hasn't been any change to Part 21, so it appears that there still will be Type Certificates, and per 21.20 you stll need to show compliance to the relevant standards, which for normal Category is Part 23.
The big change to Part 23 is that it is more performance based instead of design based (what it must do, instead of how to do it).  So the actual means of showing compliance will need to change. I don't see DERs and DARs going away with respect to Part 23. I do expect that what they review and how they do it will likely change.
To your point, LSA receive a Special Airworthiness Certificate (based on ASTM Concensus Standards), as opposed to a Type Certificate defined in Part 21.21.

Looks like the FAA and industry has more work to do if they truly want to reduce the cost of certification for Part 23 aircraft. And no the DER/DAR are not going away because of all the other catagories of

@ChristopherGagliano  OK, Fair enough: 'DER/DAR are not going away with respect to Part 23' - see edit.  Also, the FAA Aircraft Certification Service (AIR) is undergoing a 'Transformation'. It's a nice concept, but any change in an organization that size will not happen quickly or easily.


text:
What exactly are "clipped" wings?
What exactly does "clipped wing" mean? Been having a rather heated debate. My understanding is that clipped means the tips were removed after manufacture. The other party claims that if it was designed squared off and built that way, it is called clipped.

I have only ever seen the term used with wings that were shortened after manufacture.

As far as I can find you may both be correct. The term originally was used to describe a stock aircraft having it's wings shortened after-market.
Here is a reproduction of the December, 1970 issue of SPORT AVIATION Magazine article comparing stock vs clipped
http://home.xcountry.tv/~dann/id72.htm
This modification has become well known and the clipped wing design sought after that there are planes (at least kits) being manufactured with "clipped" wings.
http://www.monocoupe.com/
It appears that whether is was done 'aftermarket' or manufactured that way, clipped wings are those that have been shortened from original design to improve aerobatic capabilities.
Hopefully that will give you some more points to debate over!
~Hoff

The term clipped wings can mean many things. As @Fodder Hoff said, sometimes aircraft have their wings clipped after manufacture, but sometimes variants of certain aircraft are manufactured with clipped wings. A common example of this is the supermarine spitfire. The first Spitfire models had beautiful elliptical wings that were not clipped. Then, starting with the Spitfire Mk.V (Mark 5), some had their wings clipped to increase the roll rate and low altitude performance of the fighter. This was done because the Focke-Wulf-190 outperformed the Spitfire in those two aspects.

The term "clipped" far pre-dates 1970 and generally--as in the Spitfire context-- means a design modification that created a wingspan shorter than the original design.  It generally does not mean that someone came along with a chainsaw and shortened the wings after the aircraft was already built-- although in the context of modern civil aviation, occasionally that is exactly what it means.


text:
What forces act on cascade-type thrust reverser blocker doors?
What is the stress or force that loads the cascade-type thrust reverser blocker doors? As we know, blocker doors are closed and block the airflow going through the C duct. The engine is a V2500 with 25000 lbs thrust.

Is this a homework question?

ATTENTION, very coarse analysis !
This is a bit vague, but maybe this will give you a starting point.
Generally on turbofans, about 70% of the total thrust is provided by the fan.
If you consider a high power brake using 80% of the max thrust, this gives you
TADAAAAA 25000*0.8*0.7 = 14000 lbs (yup that's rocket science).
Now, this gives an approximation of the power provided by the fan. However, the outlet angle is not parallel to the main flow. Assuming that the mass flow rate as well as the magnitudes of the velocities are conserved in the transfer and that the outlet angle is about 45° from the engine axis, it gives
qm x (v2-v1) = F where qm is the mass flow rate, v2 the outlet velocity vector and v1 the inlet velocity vector, while F is the force of the fluid on the structure.
Because of the angles this gives qm x (0.5+1) x v1 = FX and qm x 0.5 x v = FY on the doors.
Then, assuming (yes, I said to pay attention at the beginning of my answer) that Ffan = qm*V1 = 14000 lbs, which is quite doubtful unfortunately, we obtain FX = 21000 lbs and FY = 7000 lbs.
Regarding stresses, it is IMHO far to dependent on the actual parts of the engine to assess.
If anyone can check this to be sure, be my guest ;).


text:
How much speed is required to achieve lift on something that weighs 1000lbs?
I was wondering how much speed would be needed to get an object that weighs about 1000 pounds to generate enough lift to get off the ground and achieve flight?

None, you can achieve lift at zero speed.

@RonBeyer considering that the lift formula I know has the speed as a multiplicative term, I am curious how you would achieve that. could you please write an answer?

@Federico My guess: helicopter.  Technically they can have lift when they have zero speed.

@CortAmmon you too: write an answer not a comment.

@Jon, your question shows no sign of research and has the appearance of "Let me ask someone to write an answer to save me looking it up." You could improve the question considerably by explaining (in the question) what you understand and where you are stuck.

@Federico actually I'm thinking about lighter than air vehicles.

88 miles per hour, if you've kept your flux capacitor in good repair.

Zero, if that 1000 lbs is a bag full of helium :-)  Or if it's a rocket engine and fuel tanks.  But if it's say a 1000 lb block of concrete... well, good luck with that :-)  (Downvoted for being "not even wrong".)

This is someone that is apparently new to aerodynamics. Instead of pounding them into the dirt, why not try to help them?

Lift Formula For Fixed Wing
You need more information than just speed. The lift formula shows all the variables that must be known in order to calculate lift.

Most countries require pilots to know the Lift Formula by heart and is part of their written and oral test:

L = (lb) lift
CL = (dimensionless) co-efficient of Lift
p = (1/2 rho) air density
v = (mph) velocity squared
s = (sq/ft) surface area of the wing

Lift is interdependent to all these values - so you need to know all of them.  For any given speed, the lift will increase with denser air, larger wing area, or higher angle of attack (without stalling).
For example, a 1000lb aircraft traveling at 100mph will have more lift at a lower altitude with dense air than the same aircraft at the same speed at an altitude 5000ft higher and less dense air.

Re "... a 1000lb aircraft traveling at 100mph will have more lift...", according to your formula (and common sense!) lift does not depend on weight.  Make a duplicate of a wing out of solid lead, and it will produce exactly the same amount of lift.  The really important factors are hidden within your C "variable" (really a complicated function of shape & angle of attack),

Where did I say weight was a factor in producing lift? I was comparing all things being equal except air density...

In the line I quoted about the 1000 lb aircraft.  The important point here - and why the OP's question is not even wrong - is that lift is independent of weight.  A 10,000 lb aircraft with the same shape, travelling at the same speed, altitude, &c, will produce exactly the same amount of lift.  Of course the lift produced may be insufficient for flight, but that's a different issue :-)


text:
Who manages air traffic during Red Bull air races?
I'll preface this by saying I'm not a pilot so I don't know much about Air Traffic Control.
How does Air Traffic Control work for Red Bull air races? Does:
(1) each plane taking off need to contact the local airport's ATC and let them know what they plan on doing/request permission for it, or
(2) do the event organizers schedule a block of time and airspace with ATC and take care of managing that block by themselves?
The latter seems feasible from my (non-pilot) perspective:

there's only one plane at a time on course (easy to manage)
it's a pretty well defined area that they'll be staying within.

It would help the event by:

perhaps allowing some of the long-range communication systems to be removed from the airplanes (reducing weight - useful for racing)
give the pilots fewer things to think about prior to their run on the course
transferring the control of take-offs and landings to the organizers makes it easier for them to stay on schedule

I think there would be advantages for them to do it the second way, and I think it would be technically possible to safely do it that way. However, it's not what I think is "standard procedure" and I wonder if they strayed from standards for their events' organization.
So are they doing it the first or second way? If they're doing it the first way, why? It seems inefficient, could they do it the second way (legality/regulation -wise)? Did I miss something that would make method (2) more trouble than it's worth? Or maybe they're doing it some other way I didn't think about?

This is a fairly broad question since the races move around the world the the regulations vary heavily depending on where its occurring but ill answer for the FAA understanding that most countries treat this kind of stuff similarly. The Air race its self has its own control tower of sorts that talks directly to the pilots to clear them onto and off of the course (this seems to be #2 in your question) but local ATC will be involved for departures.
Depending on where they intend on flying they may need to get a special flight permit, these are applied for in advance with the FSDO. I dont know if this is something the teams need to do or if this is something organizers handle. If the country has such regulations a TFR may be issued, for example this one from the french race (thanks to @mins for digging it up!). This keeps traffic out of the way so the organizers don't to worry about a Cessna 150 putting through the course @90Kts trying to "watch the race".
The planes typically depart from a local field close to the course. Here in the US they need to follow the same procedures as anyone else, if its controlled that means ground and tower coms if its uncontrolled thats CTAF announcements.
Once in the air they will head to the race course. Since these planes are effectively VFR radio com's is not mandatory outside of controlled airspace. The tower may simply transition them outside of the terminal area to the course frequency when appropriate.
According to this walk through the aircraft has one radio for taking to "race control" who clear them through the course. This behind the scenes video will walk you through how race control works and what they do.
No communication occurs during the race its self, you can see in this video the pilot is mostly concentrating on breathing and flying.
Once off the course they will either loiter around if they need to run again or head back to the airport, heading back is the same as any flight, fly on over and call up the tower to request landing.
This podcast is also a pretty great overview on the race and what it takes to be a pilot.

Excellent answer.  I have never participated in a Red Bull air race, but have been in a few military flyovers of public events.  I will just add that typically a TFR, (temporary flight restriction) will be published for the affected area, and participants will be assigned a transponder code so that ATC can positively identify those authorized to enter the TFR in order to help keep others out.  Once you are inside that area the communication/control of the aircraft shifts away from traditional ATC services and responsibility.

@MichaelHall I had thought about that as well, i could not find any reports for the Red Bull TFR (which of course does not preclude their existence) and without a link to one I did not want to add it.

I think a lot of races are in Europe where maybe they have less GA traffic to worry about?  Anyhow, I would imagine that a similar event in the states would most likely have a TFR associated with it.  Just an educated guess...

@mins if you dont mind ill add that doc to the answer.


text:
Can a non FAA CFI (EASA for eg) sign off a high performance endorsement on an FAA PPL certificate?
I have an FAA PPL cert, flying in the UK. Can a qualified EASA CFI sign off a high performance endorsement on my FAA certificate.
Thanks

Welcome to Aviation SE!  This is a very good first question.

No, the problem is that the endorsement must come from an "authorized instructor" which is defined in 61.1(b):

Authorized instructor means -
(i) A person who holds a ground instructor certificate issued under part 61 of this chapter and is in compliance with § 61.217, when conducting ground training in accordance with the privileges and limitations of his or her ground instructor certificate;
(ii) A person who holds a flight instructor certificate issued under part 61 of this chapter and is in compliance with § 61.197, when conducting ground training or flight training in accordance with the privileges and limitations of his or her flight instructor certificate; or
(iii) A person authorized by the Administrator to provide ground training or flight training under part 61, 121, 135, or 142 of this chapter when conducting ground training or flight training in accordance with that authority.

The requirement for an "authorized instructor" comes from 61.31:

(f)Additional training required for operating high-performance airplanes.
(1) Except as provided in paragraph (f)(2) of this section, no person may act as pilot in command of a high-performance airplane (an airplane with an engine of more than 200 horsepower), unless the person has -
(i) Received and logged ground and flight training from an authorized instructor in a high-performance airplane, or in a flight simulator or flight training device that is representative of a high-performance airplane, and has been found proficient in the operation and systems of the airplane; and
(ii) Received a one-time endorsement in the pilot's logbook from an authorized instructor who certifies the person is proficient to operate a high-performance airplane.

So you can't get an endorsement from a "CFI" who is not authorized under Part 61.


text:
Can small planes see lights on the ground?
I live about 5 miles from a small airport. At night, during rain or snow storms when I imagine there would be poor visibility, I always turn our outside house lights on, in case they could possibly help a little plane in distress. Am I crazy?

You're not crazy, you're considerate.  Thanks for thinking about people in little planes.

This might be a nice gesture and the fact that you think about things like that shows your keen to learn. BUT it really is not neccessary! If visibility is an issue, the pilot is/should be flying IFR anyway. Reducing your power consumption might help the planet more...

Thanks, I appreciate that. I won't do it anymore.

They can see your lights, but they don't help.  "Black hole" approaches (where there aren't any lights on the ground except for the runway) can be difficult and unnerving, but even if that's the case for that airport, one set of house lights isn't going to appreciably affect the situation.

Thank you, Ralgha. I won't do it again.

It doesn't hurt either, and I must disagree with the other answer in that respect.  If someone confuses house lights for runway lights, they need more practice and shouldn't be flying hard IFR at night.

Yes we can see lights on the ground from small planes, but your extra lights do not help approaching aircraft. If anything they may serve to disorient pilots. In low visibility situations, provided the pilot is properly rated and the aircraft is properly equipped they may be flying IFR. Under these types of operations when popping out of the clouds we may be looking for runway lighting. Other lights on the ground can (and have been) confused for runway lights in the past.

Thanks, Dave. I'm glad I asked.


text:
What does the term "loading" refers to when we talk about Blade loading and Stage loading in Turbomachinery?
I have been reading about the term Blade Loading and Stage loading in case of Axial Turbines and Compressors, however, I would like to know what does the term "loading" here exactly refers to?
What I also read was that if we increase the number of blades then the Blade loading for each blade will reduce but the friction and weight will increase, then what is the optimum number of blades one should have?

The term “blade loading” is basically the same as “pressure ratio”. A blade with a higher blade loading causes the exit pressure from that blade stage to be higher than if the blade loading was lower. Hence, the pressure ratio (exit pressure/inlet pressure) is higher when the blade loading is higher.
The optimum number of blades is a very complex problem. But, in simple terms, too many blades, and the weight is too much, too few blades, and either the exit pressure of the compressor is lower (which makes the engine less fuel efficient), or the pressure ratio across each blade is higher, and the engine is much more likely to suffer compressor stalls. The fuel control may have to allow only a very slow acceleration and deceleration rate, to avoid stalls. It may also stall more easily in crosswind situations. Neither of these are ideal in a take-off situation. You don’t want the engine coughing and losing thrust when you are past the point of aborting the take-off, but aren’t high enough to fly a circuit and land.